2.1. Protein Kinase Cascades and the Regulation of Cell Function
A cascade of phosphorylation reactions, initiated by a receptor tyrosine kinase, has been proposed as a potential transducing mechanism for growth factor receptors, including the insulin receptor (Cobb and Rosen, 1984, Biochim. Biophys. Acta. 738:1-8; Denton et al., 1984, Biochem. Soc. Trans. 12:768-771). In his review of the role of protein phosphorylation in the normal control of enzyme activity, Cohen (1985, Eur. J. Biochem. 151:439-448) states that amplification and diversity in hormone action are achieved by two principal mechanisms, the reversible phosphorylation of proteins and the formation of "second messengers"; many key regulatory proteins are interconverted between phosphorylated and unphosphorylated forms by cellular protein kinases and certain protein phosphatases.
Some hormones appear to transmit their information to the cell interior by activating transmembrane signalling systems that control production of a relatively small number of chemical mediators, the "second messengers." These second messengers, in turn, are found to regulate protein kinase and phosphatase activities, thereby altering the phosphorylation states of many intracellular proteins, and consequently controlling the activity of enzymes which are regulated by their degree of phosphorylation (see FIG. 1). The receptors for other hormones are themselves protein kinases or interact directly with protein kinases to initiate protein kinase signalling cascades. These series of events are believed to explain the diversity associated with the actions of various hormones (Cohen, 1985, Eur. J. Biochem. 151:439-448; Edelman et al., 1987, Ann. Rev. Biochem. 56:567-613).
Insulin, like most cellular regulators, exerts its effects on many cellular processes through alterations in the phosphorylation state of serine and threonine residues within regulated proteins. Insulin exerts these effects via its receptor, which has intrinsic tyrosine-specific protein kinase activity (Rosen et al., 1983, Proc. Natl. Acad. Sci. U.S.A. 80:3237-3240; Ebina et al., 1985, Cell 40:747-758). Of note, the proteins encoded by several oncogenes are also protein-tyrosine kinases. For example, P68.sup.gag-ros, a transmembrane transforming protein, bears many similarities to the insulin receptor, sharing 50% amino acid identity (for discussion, see Boulton et al., 1990, J. Biol. Chem. 265:2713-2719).
Nerve growth factor (NGF), a neurotrophic agent necessary for the development and function of certain central and peripheral nervous system neurons, is also believed to influence cellular functions, at least in part, by altering phosphorylation of intracellular proteins. It has been observed that NGF promotes changes in the phosphorylation of certain cellular proteins (discussed in Volonte et al., 1989, J. Cell. Biol. 109:2395-2403; Aletta et al., 1988, J. Cell. Biol. 106:1573-1581; Halegoua and Patrick, 1980, Cell 22:571-581; Hama et al., 1986, Proc. Natl. Acad. Sci. U.S.A. 83:2353-2357; Romano et al., 1987, J. Neurosci, 7:1294-1299). Furthermore, NGF appears to regulate several different protein kinase activities (Blenis and Erikson, 1986, EMBO J. 5:3441-3447; Cremins et al., 1986, J. Cell Biol. 103:887-893; Landreth and Rieser, 1985, J. Cell. Biol. 100:677-683; Levi et al, 1988, Mol. Neurobiol. 2:201-226; Mutoh et al., 1988, J. Biol. Chem. 263:15853-15856; Rowland et al., 1987, J. Biol. Chem. 262:7504-7513). Mutoh et al. (1988, J. Biol. Chem. 263:15853-15856) reports that NGF appears to increase the activities of kinases capable of phosphorylating ribosomal protein S6 (S6 kinases) in the PC12 rat pheochromocytoma cell line, a model system regularly used to study NGF function. Volonte et al. (1989, J. Cell. Biol. 109:2395-2403) states that the differential inhibition of the NGF response by purine analogues in PC12 cells appeared to correlate with the inhibition of PKN, an NGF-regulated serine protein kinase. Additionally, activators of the cyclic AMP dependent protein kinase (PKA) and protein kinase C (PKC) have been reported to mimic some but not all of the cellular responses to NGF (Levi et al., 1988, Mol. Neurobiol. 2:201-226). Miyasaka et al. (1990, J. Biol. Chem. 265:4730-4735) reports that NGF stimulates a protein kinase in PC12 cells that phosphorylates microtubule-associated protein-2. Interestingly, despite the many reports linking NGF with changes in phosphorylation of cellular proteins, analysis of a cDNA sequence encoding a subunit of the NGF receptor which is sufficient for low-affinity binding of ligand has indicated no evidence for a protein-tyrosine kinase domain in the cytoplasmic region of this low affinity receptor (Johnson et al., 1986, Cell 47:545-554).